1. Field of the Invention
The present invention relates to an image-retransferable sheet having a layer of a surface treating agent and more particularly to an image-retransferable sheet to which an image including letters, symbols and figures are thermally printed by a heat-sensitive image-transferring type printer and from which the thermally transferred image are retransferred onto a surface of an image-receiving material with pressure.
2. Description of the Prior Art
It is known to produce graphic images including letters, symbols and figures by a thermal transferring head equipped in a heat-sensitive image-transferring type printer. The thermal printer uses a thermal ink ribbon and a thermal printer head which presses the thermal ink ribbon and applies heat thereon in order to transfer an image from the thermal ink ribbon onto a recording sheet. The ink ribbon is disclosed, for example, in U.S. Pat. No. 4,778,729. However a transferred image of the thermal ink ribbon disclosed in U.S. Pat. No. 4,778,729 can be formed on the recording sheet which is used in the thermal printer and cannot be used for an image-retransferable sheet to which an image including letters, symbols and figures are thermally printed and from which the thermally transferred image are retransferred onto a surface of an image-receiving material by pressure.
Tape printers, such as those sold under the mark P-Touch by Brother Industries Co., Ltd., have been developed. Such tape printers employ an image-retransferable sheet, such as those disclosed in U.S. Pat. No. 4,870,427 and corresponding unexamined and published Japanese patent application No. 63-128987. Such image-retransferable sheets have a substrate 331 and a release layer 332 supported thereon as shown in FIG. 3B. In the image-retransferable sheet 33, fluorine-based resin films, papers, coated papers, metal foils or plastic films of polyethylene or polypropylene are used as the substrate 331 and the release layer 332 is coated on the substrate 331.
As shown in FIG. 3A, tape printer 30 uses a thermal ink transfer ribbon 31 and a thermal printer head 32 mounted in the tape printer 30. In the tape printer 30, the thermal printer head 32 presses the thermal ink ribbon 31 onto the image-retransferable sheet 33 and applies heat thereon in order to transfer a fused thermal ink image 34 onto the image-retransferable sheet 33. Therefore, the ink image 34 of the ink layer is formed on the release layer 332 of the image-retransferable sheet 33 as shown in FIG. 3B.
Next, the user of the tape printer 30 arranges the image-retransferable sheet 33 so that the ink image 34 and a front side 33A of the image-retransferable sheet 33 face and are placed in close contact with an image receiving member 35. An image of the fused ink 34 is transferred onto the image receiving member 35 by application of pressure to a back side 33B of the image-retransferable sheet 33, in a manner that the image of the fused ink 34 is separated from the front side 33A of the release layer 332 and the fused ink 34 is transferred to a front face of the image receiving member 35. As shown in FIG. 3C, the user forms the image of the fused ink 34 on the image receiving member 35 which the user desires to form thereon.
In the above transferring process from the ink ribbon 31 to the image-retransferable sheet 33 by heat of the thermal head 32, the image of the fused ink 34 is required to be transferred onto the image-retransferable sheet 33 steadily and in the above retransferring process from the image-retransferable sheet 33 to the image receiving member 35 by application of pressure, the image of the fused ink 34 is required to be retransferred onto an image receiving member 35 steadily. To obtain the steady transfer and retransfer, an image-retransferable sheet 33 having the release layer 332 comprising a silicone resin, the release layer 332 having a smooth surface thereon and exhibiting a contact angle with water of at least 95.degree., has been developed.
However, in order to thermally print an image onto the image-retransferable sheet 33 having a water contact angle of at least 95.degree. and particularly not less than 105.degree., it is necessary to reduce the surface tension of the fused ink 34 to wet the sheet 33 and it is further necessary to increase adhesion between the fused ink 34 and the sheet 33 to a level greater than the cohesive force between the fused ink 34 and a base layer of the thermal transfer ribbon 31, such as polyethylene terephthalate film. For the purpose, the temperature of the fused ink 34 must be increased in order to reduce the surface tension of the fused ink 34 when the image is thermally printed on the sheet 33. Therefore, high energy must be applied to the thermal head 32 which is disadvantageous in view of durability of the thermal head 32 and load on a power supply.
Further, ink images formed on a sheet 33 having poor wettability are easily retransferred with little pressure because of poor adhesion to the sheet 33, and a portion of the image which is desired to be left on the sheet 33 is undesirably retransferred to the image receiving member 35, causing stains on the image-receiving member 35. Such easy retransfer is troublesome in handling of the sheet 33.
Furthermore, since the sheet 33 has an extremely small static friction coefficient, the sheet 33 is not easily fixed during retransfer of the thermally printed image from the sheet 33 to the image-receiving member 35, so that the image of the fused ink 34 is retransferred on an undesired portion of the image-receiving member 35 or distorted on the image receiving member 35.
Surface treating agents in the release layer 332 are used in conventional image-retransferable sheets 33 to improve the property of retransferring an ink image from the sheet 33, so that the user can easily retransfer the fused ink 34 on the sheet 33 to the image receiving member 35. However, the release layer 332 is not transferred to an image-receiving member 35 with the image of the fused ink 34. Therefore, the image-retransferable sheet 33 necessarily must have poor wettability, to reduce adhesion between the sheet 33 and the ink image of the fused ink 34. Because of the features of such an image-transferable sheet 33, however, the sheet 33 suffers from various problems, such as reduced capability in receiving an ink image thermally printed or transferred from an ink ribbon 31, increased energy needed for thermally transferring an ink image, and poor resistance to friction of an ink image thermally transferred.